1. Field of Invention
The present invention relates to a comparator circuit, a pulse-width modulation circuit, an electrooptic device, and an electronic apparatus suitable for application to various information displays.
2. Description of Related Art
Electrooptic devices, for example, liquid crystal display devices that each include a liquid crystal as an electrooptic material are widely used as display devices for display sections of various information processing apparatuses, liquid crystal televisions, etc. instead of cathode ray tubes (CRT). A related art electrooptic device requires a main body unit (for example, personal computer) which constantly supplies image signals for display because the electrooptic device itself cannot store information. This technique causes the main body unit to continuously output the same image signal for displaying a static image in the electrooptic device. Even when a moving image is displayed, in many cases, gradation data is only changed in a part of pixels in a frame period. Therefore, when the main body unit continuously outputs image signals over the entire screen, electric power is wasted.
Thus, the inventors proposed an electrooptic device including a memory with a few bits provided for each of pixels so that gradation data of each pixel is maintained for each pixel, and image display is continued even when no image signal is input from the outside, as disclosed in Japanese Application No. 2000-270424. This prior application was not laid open to public inspection at the time that this application was filed. In the electrooptic device of the prior application, 8-gradation data is stored in a 3-bit memory for each of the pixels. Also, a so-called sub-field driving system is used as a driving system. Namely, either an on-state potential VH or off-state potential VL is applied to a liquid crystal layer, and the duty ratio of the on-state potential VH is set according to the gradation data.
In the prior application, the duty ratio of the on-state potential VH is set according to the gradation data, and thus gradation signals resulting from cyclic counting of predetermined clock pulses in a certain range (for example, “0” to “7”) are supplied to each of the pixels. Also, a comparator is provided on each of the pixels in order to compare the gradation data and the gradation signals. If the count values of the gradation signals are less than the gradation data stored in the memory, the on-state potential VH is selected as the potential applied to the liquid crystal layer. If the count values of the gradation signals exceed the gradation data stored in the memory, the off-state potential VL is selected. In this construction, the effective value of the applied voltage can be set according to the gradation data to permit a gradation display. The cycle of the clock pulses is not constant, and is appropriately changed to compensate the nonlinearity of a liquid crystal.